This invention relates generally to global positioning system ("GPS") satellite signal receivers, and, more particularly, to improvements in their digital processing sections.
The United States government is in the process of placing into orbit a number of satellites as part of a global positioning system (GPS). Some of the satellites are already in place. A receiver of signals from several such satellites can determine very accurately parameters such position, velocity, and time. There are both military and commercial uses. A primary military use is for a receiver in an aircraft or ship to constantly determine the position and velocity of the plane or ship. An example commercial use includes accurate determination of the location of a fixed point or a distance between two fixed points, with a high degree of accuracy. Another example is the generation of a high accuracy timing reference.
In order to accomplish this, each satellite continually transmits two L-band signals. A receiver simultaneously detects the signals from several satellites and processes them to extract information from the signals in order to calculate the desired parameters such as position, velocity or time. The United States government has adopted standards for these satellite transmissions so that others may utilize the satellite signals by building receivers for specific purposes. The satellite transmission standards are discussed in many technical articles and are set forth in detail by an "Interface Control Document" of Rockwell International Corporation, entitled "Navstar GPS Space Segment/Navigation User Interfaces", dated Sep. 26, 1984, as revised Dec. 19, 1986, hereinafter referred to as the "ICD-GPS-200".
Briefly, each satellite transmits an L1 signal on a 1575.42 MHz carrier, usually expressed as 154 f0, where f0=10.23 MHz. A second L2 signal transmitted by each satellite has a carrier frequency of 1227.6 MHz, or 120 f0. Each of these carrier signals is modulated in the satellite by at least one pseudo-random signal function that is unique to that satellite. This results in developing a spread spectrum signal that resists the effects of radio frequency noise or intentional jamming. It also allows the L-band signals from a number of satellites to be individually identified and separated in a receiver.
One such pseudo-random function is a precision code ("P-code") that modulates both of the L1 and L2 carriers in the satellite. The P-code has a 10.23 MHz clock rate and thus causes the L1 and L2 signals to have a 20.46 MHz bandwidth. The P-code is seven days in length. In addition, the L1 signal of each satellite includes a carrier in phase quadrature with the P-code carrier that is modulated by a second pseudo-random function. This second modulating function is a unique clear acquisition code ("C/A-code") having a 1.023 MHz clock rate and repeating its pattern every one millisecond, thus containing 1023 bits. Further, the L1 carrier is also modulated by a 50 bit-per-second navigational data stream that provides certain information of satellite position, status and the like.
In a receiver, signals corresponding to the known pseudo-random P-code and C/A-code may be generated in the same manner as they are in the satellites. The L1 and L2 signals from a given satellite are demodulated by aligning the phases of the locally generated codes with those modulated onto the signals from that satellite. The relative phases of the two carriers may then be determined. The carrier signal phases and pseudo-range measurements from a number of satellites are measurements that are used by a receiver to calculate the desired end quantities of distance, velocity, time, etc. The apparent transmission time of the signals from a given satellite to the GPS receiver can be measured, from which an apparent range to that satellite may be computed.
The C/A-code modulated phase quadrature carrier component of the L1 signal is provided for commercial use. If the accuracy desired in the quantity being measured by the receiver is not great, use of the L1 signal carrier alone is satisfactory. However, for applications where high resolution measurements are desired to be made, and/or the measurements must be made quickly, the L2 carrier must also be used. The measurement becomes more accurate by eliminating an unknown delay of the signals by the ionosphere when both of the L1 and L2 signal carriers are used.
Although the P-code functions of all the satellites are also known, the satellites are provided with means to modulate the P-code with a secret signal in order to prevent jamming signals from being accepted as actual satellite signals. This "anti-spoofing" allows the GPS system to be used for military or other sensitive United States Government applications. The secret modulating signal, often referred to as the "A-S code" and designated herein for convenience as the "A-code", may be turned on or off at will by the United States government. When on, according to the ICD-GPS-200, the P-code is replaced by a Y-code on both the L1 and L2 carriers. It has been disclosed publicly that the Y-code is the modulo-two sum of the known P-code and the unknown A-code. In order to be able to extract the carrier from an anti-spoofed L2 signal by the straight-forward demodulating technique described above, the Y-code or A-code would have to be known. Since the A-code is classified by the United States Government, such L2 signal demodulation cannot be accomplished by commercial GPS receiver manufacturers or users.
As a result, other techniques have been suggested to obtain the L2 signal carrier. One such "codeless" technique is to square the received L2 signal, thus eliminating its modulating terms. This is utilized in the receiver described in U.S. Pat. No. 4,928,106--Ashjaee et al (1990). Although satisfactory for many applications, the squaring of the spread spectrum signal causes the signal-to-noise ratio to be degraded. Alternatively, the modulation may be removed by multiplying the upper and lower sidebands of the L2 carrier signal as described in U.S. Pat. No. 4,667,203--Counselman (1987).
In order to reduce this signal-to-noise degradation, it has also been suggested to adjust the phase of a locally generated replica of the known P-code until a strong demodulated signal appears out of the noise. This narrower bandwidth signal is then squared in order to eliminate the unknown modulation without hurting the signal-to-noise level as much as when the entire L2 signal is squared. Such a technique is described in U.S. Pat. No. 4,972,431--Keegan (1990).
However, the technique described in the Keegan patent results in a half wavelength L2 carrier phase observable, making it more difficult to quickly resolve integer ambiguities. Also, the signal-to-noise ratio resulting from the technique of the Keegan patent is not optimal. It is, therefore, a primary object of this invention to provide a technique of processing GPS satellite signals that overcomes these limitations.
It is a more general object of the present invention to provide a technique for using carrier signals modulated by the anti-spoofing A-code without having to know the A-code.
It is another object of the present invention to provide a technique of determining relative phases of GPS satellite signals with an increased degree of resolution.
It is a further object of the present invention to provide an improved radio frequency front end section for a GPS receiver.